Hello. I am working on a horror game and was wondering what is the best and quickest way to make an indoor layout. Doorways are particularly difficult because they require three cubes to make just using unity’s primatives. I just want to have an easy way to map out the basic layout of the level that I will then replace with meshes and materials later. Thanks.
Once you create your basic doorway from 3 cubes though, you just turn it into a prefab for reuse. It literally doesn’t get easier than that.
You might like something along the lines of Pro Builder for putting your levels together:
Okay. That is very helpful. Thanks!
One question. I drag one cube into the prefab and it changes to cube into a prefab but I can’t add any more objects to the one prefab. Is there any way to merge objects with one another in a single GameObject so I can turn that into a prefab instead of several smaller objects?
I think you might be approaching this the wrong way 
In general, what you’re trying to do is “model” objects (e.g. your doorframe). You generally wouldn’t do that within Unity itself, but rather you would use a 3D modelling application and then import your models into Unity.
The alternative is to use something like Pro Builder which gives you modelling capabilities directly within Unity.
To answer your question, no - there’s no easy way to merge say 3 cubes into a single mesh within Unity itself. You could however place the 3 cubes in their arrangement under an empty parent GameObject (called “Doorframe” for example), and then create a prefab of that parent GameObject in order to make millions of little baby doorframes.
Alright then. Thanks for your help!